hive-c0re: fail on a malformed agent name and on an unreadable container list

An agent name that is not a valid Ident made `Coordinator::agent_paths`
panic. Job payloads carry names as plain strings (the swarm's published
wanted state is one source), and a panic inside a job-queue node never
reaches `complete_growing`, so the node's resources (the deploy
window included) were held until hive-c0re restarted. `agent_paths` now
returns an error; the job-queue nodes, the admin-socket spawn and
set-limits paths, the root-agent spawn and the dashboard set-limits
handler propagate it.

`lifecycle::list().await.unwrap_or_default()` turned a failed container
list into "no agents":
- meta-update cascade: the lock bump committed and zero rebuilds fanned
  out, reported as success. The cascade is now resolved before the lock
  bump and a list failure fails the node.
- dashboard update-all: queued nothing and returned 200 "ok". Now 500
  with the error.
- container rescan: every row was emitted as removed and the cache
  emptied. Now the last snapshot stands; `hivectl status` gets an error.
- dashboard journal: answered 404 "no managed container". Now 500.
- spawn/rebuild port-collision check: silently skipped. Now fails.
- startup migration: the per-agent phases ran over nothing, and phase 3
  handed an empty agent list to `meta::sync_agents`, which renders the
  meta flake with exactly the agents it is given. Both now log the list
  failure and skip.

The hive-jobq scheduler still leaks a node's resources on any executor
panic; that root is not addressed here.

Refs #4723

hive-c0re/src/dashboard/lifecycle_ops.rs

@@ -392,7 +392,10 @@ pub(super) async fn post_resource_limits(
     }
     let agent_dir = crate::paths::agent_runtime_dir(ident.as_str());
     let hive = state.coord.hive_env();
-    let paths = crate::coordinator::Coordinator::agent_paths(ident.as_str(), agent_dir);
+    let paths = match crate::coordinator::Coordinator::agent_paths(ident.as_str(), agent_dir) {
+        Ok(p) => p,
+        Err(e) => return error_response(&format!("write_dropins {logical}: {e:#}")),
+    };
     if let Err(e) = crate::lifecycle::write_dropins(ident.as_str(), &hive, &paths).await {
         return error_response(&format!("write_dropins {logical}: {e:#}"));
     }
@@ -405,18 +408,18 @@ pub(super) async fn post_resource_limits(
 #[utoipa::path(
     post,
     path = "/api/update-all",
-    responses((status = 200, description = "rebuilds queued", body = String)),
+    responses(
+        (status = 200, description = "rebuilds queued", body = String),
+        (status = 500, description = "container list unreadable, nothing queued"),
+    ),
     tag = "lifecycle_ops"
 )]
 pub(super) async fn post_update_all(State(state): State<AppState>) -> Response {
-    let containers = lifecycle::list().await.unwrap_or_default();
-    for container in containers {
-        let Some(logical) = container
-            .strip_prefix(lifecycle::AGENT_PREFIX)
-            .map(str::to_owned)
-        else {
-            continue;
-        };
+    let agents = match lifecycle::agent_names(lifecycle::list().await) {
+        Ok(agents) => agents,
+        Err(e) => return error_response(&format!("update-all: listing containers: {e:#}")),
+    };
+    for logical in agents {
         if let Err(e) = state.coord.job_queue.insert_job(|b| {
             crate::job_queue::templates::rebuild(b, &logical, true);
             Vec::new()